As efforts are made to decrease the production of polluting nitrogen oxides from gas turbine engines, use is made of so-called lean burn pre-mix combustors in which the fuel to air ratio is reduced as far as possible in the higher operating range. This has disadvantages which the present invention seeks to reduce. Firstly, in combustors with radial inflow pre-mixing burners which impart a high degree of swirl to a primary lean fuel/air mixture before feeding it into an axial flow pre-chamber in flow series with a main combustion chamber, a re-circulating vortex core flow of hot combustion gases, extending between the burner and the main combustion chamber, can impinge on the burner face, leading to high surface temperatures which may reduce the working life of the component material in that region. Secondly, the weak fuel/air mixture leads to a problem in maintaining flame stability when the engine load is reduced, leading to the need to use fuel-rich pilot-flame systems or other means for changing the fuel/air ratio at low engine loads. Such approaches typically lead to an increase in harmful emissions, and may require a more complicated and expensive design of combustor.